Such devices for the surface finishing of workpieces with the use of a granular grinding and/or polishing medium are known, for example, in the form of so-called dragging and dipping finishing machines. Their mode of action is based on the fact that the workpiece to be finished is dipped into a bed of granular grinding and/or polishing material, which is contained in a container, and the workpiece is moved relative to the granular material, especially by a rotary motion as well as optionally by translatory motion, as a result of which the surface of the workpiece is ground and/or polished, depending on the type of the granular material. Drag-finishing and dip-finishing machines represent a special form of slide-grinding machines, wherein the workpieces to be finished can be clamped, e.g., individually, on one or more workpiece holders rotatable about its/their axis by means of a rotary drive. To move the workpieces by a translatory motion relative to the granular grinding and/or polishing material, prior-art drag-finishing machines often comprise a usually rotating part essentially in the form of a plate, which is driven rotatingly, e.g., by a motor via a suitable gear mechanism, and on which the workpiece holders are fixed directly or indirectly, e.g., via lifting device. This happens especially eccentrically in relation to the axis of rotation of the rotating part of the drag-finishing machine. If this part, the so-called plate, of the drag-finishing machine is rotated, the workpiece holders fixed thereon describe a trajectory. The workpieces being carried by the workpiece holders are now dipped into the working container, which is filled with the bed of particulate granular grinding or polishing material, often with liquid finishing media, such as water, surfactants, etc., added. Based on the relative motion of the workpieces in relation to the granular material, the surface finishing of the workpieces takes place in the form of a slide-grinding finishing. Such drag-finishing machines are known, for example, from DE 102 04 267 C1, DE 200 05 361 U1 or DE 10 2010 052 222 A1.
DE 10 2011 103 606 A1 and DE 10 2009 021 824 A1 show additional devices for the surface finishing of large-sized workpieces, especially in the form of turbines, which comprise a rotatingly driven workpiece holder, which dips into a stationary container containing the bed of granular material. A moving unit associated with the workpiece holder ensures a rotary or alternating motion of the workpiece about the axis thereof or even different motions (rotating and oscillating motions) of the workpiece in the granular grinding material with the speed of rotation and the dipping depth being varied.
As an alternative or in addition to a translatory motion of the workpieces themselves clamped on the workpiece holders, the container containing the finishing medium may also be moved relative to the workpieces, which are, for example, rotated themselves at least about their own axis, for example, about its own axis and/or along a trajectory, e.g., in the form of a circular path. If only the container is moved and the workpieces themselves do not perform any translatory motion, this is also called “plunge-cut grinding,” which thus represents a special form of drag-finishing.
To bring about automated operation, modern devices usually used for the surface finishing of the workpiece, such as the dip- and/or drag-finishing machines of the aforementioned type, comprise, as a rule, a program-based control device, which is capable of controlling at least the rotary drive of the workpiece holders as well as optionally also a translatory drive thereof and/or a (rotary) drive of the container according to different speeds of rotation, rates of motion and finishing times, which can be inputted in an input device.
DE 10 2011 015 750 A1 describes another method and a device for the surface finishing of workpieces, which is provided especially for the gentle finishing of very delicate workpieces, especially in the form of optical lenses, which must meet very narrow tolerances. The workpiece holders are carried in this case by a manipulator controllable in a programmed manner, such as an industrial robot, in order to pivot the workpiece clamped on the workpiece holder during the finishing about a plurality of different axes and to make it possible in the process to change the angle of impact at which the granular grinding and/or polishing material being moved in the container comes into contact with discrete surface areas of the workpiece and thus to ensure the desired grinding and/or polishing effect for the particular surface area of the workpiece. The manipulator is capable of rotating the workpieces clamped on the workpiece holder about the axis of the workpiece holder continually, as desired, at a constant speed of rotation or at adjustably different speeds of rotation and/or directions of rotation and/or of moving same by a translatory motion in the bed of the granular grinding and/or polishing material or even to only dip the workpiece into said bed. The container itself may comprise a controlled rotary drive in order to set it into rotation at the particular, desired speed of rotation and to change the velocity at which the granular materials meets the workpiece.
The granular grinding and/or polishing material used in grinding and polishing methods of this class may also differ, in principle, depending on the workpieces to be finished and may be, e.g., of natural origin (consist of, e.g. organic material, such as walnut or coconut shells, wood, cherry stones, etc.), of inorganic origin (consist, e.g., of silicates, oxides, etc.) and/or of synthetic origin (consist of, e.g., plastics). Moreover, it is possible, as was already indicated, to carry out the slide-grinding finishing dry or, with the addition of a liquid finishing medium, for example, water, which may be mixed with additives, e.g., surfactants, in the form of a wet finishing.
It was, however, found that unsatisfactory surface finishing of the workpieces may occur in at least some areas, which applies especially to workpieces with a relatively more complex geometry, such as one with grooves, undercuts or even with cavities of varying sizes, depressions or the like, even in case of a rotary motion of the workpieces about the axis of the workpiece holder, which may optionally occur both clockwise and counterclockwise, i.e., in different directions of rotation, wherein a translatory motion of the workpieces through the granular grinding and/or polishing material is superimposed to said rotary motion. The reason for this is mainly that the particles of the granular material are carried along by the workpiece in the immediate vicinity of the workpiece being rotated relative to the bed of granular material, so that the relative motion of the particles of granular material in relation to the workpiece is markedly lower at the surface of the workpiece than the speed of rotation of the workpiece itself (or than the speed of rotation of the bed of granular material being rotated together with the container). This is especially true, as was stated, if the workpiece is provided with surface structures, such as grooves, undercuts, etc., or if the workpiece has one or more larger cavities, in which the particles of the granular material can be deposited and carried along with the rotating workpiece.
This problem is sought to be eliminated in the state of the art mainly by the workpiece being rotated at the highest possible speed of rotation, but this requires a great effort in terms of drive technology, and the maximum speed of rotation is limited. In addition, it was found that a mere reversal of the direction of rotation is incapable of eliminating this problem completely even if the workpiece is rotated at high speeds of rotation both clockwise and counterclockwise. As far as relatively large cavities or depressions of the workpiece are concerned, as they occur, for example, in the case of workpieces in the form of bottles or other vessels or dies having a die cavity, even very high speeds of rotation are able to lead to satisfactory surface finishing of the inner walls enclosing such cavities or depressions of the workpiece only after an (excessively) long finishing time.
DE 20 2009 008 070 U1 describes a workpiece holder intended for drag-finishing machines, whose rotatable workpiece carriers, onto which the workpieces to be finished can be clamped, are arranged at a slope angle between 5° and 35° in relation to the clamping device of the workpiece holder itself, wherein the slope angle may be adjustable especially between said angle range and an angle of 0 (vertical arrangement). The deposition of particles of granular material in grooves or undercuts of the workpiece is effectively reduced in this manner, because the particles are again discharged by themselves from such surface structures of the workpiece during the operation as a consequence of the slope of the angle of rotation. However, the aforementioned problem that a velocity gradient develops here as well is likewise present here, and the relative velocity of the granular material in relation to the workpiece being rotated decreases with decreasing distance between a particle of granular material and the workpiece.